Method of making cathode current collectors for sodium sulphur cells

ABSTRACT

In a sodium sulphur cell, improved protection of a cathode current collector is obtained by isostatically pressing graphite foil or flakes onto the surface of the collector, which surface is preferably roughened or coated with an intermediate conductive layer such as a nickel-chromium alloy or a conductive carbide providing a rough surface.

This application is a continuation, of application Ser. No. 217,334,filed 12/17/80, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to sodium sulphur cells and to cathode currentcollectors for such cells and methods of making such cells and currentcollectors.

2. Prior Art

In a sodium sulphur cell, a solid electrolyte material, typically betaalumina, separates molten sodium, constituting the anode, from thecathodic reactant comprising molten sulphur and sodium polysulphides.The cathodic reactant is highly corrosive. The cathode currentcollectors must make electrical contact with the cathodic reactant andmany proposals have been made regarding the construction of such currentcollectors. Although carbon and graphite are widely used because oftheir corrosion-resistant properties, they do have a very substantiallyhigher electrical resistance than most metals. For this reason, use ismade of composite current collectors having a substrate of a relativelyhighly conductive material with an outer sheath or coating of somematerial having good corrosion-resistant properties. Such an arrangementenables an aluminium substrate to be employed giving the advantages ofthe high conductivity of that material; aluminium however if exposed tothe cathodic reactant in a sodium sulphur cell will form a sulphidecoating which is non-conductive. It is important therefore that theouter coating should protect the aluminium from any exposure to thecathodic reactant. Reference is made to U.S. Pat. Nos. 3,982,975;4,024,320; 4,061,840 and 4,219,690 as examples of composite cathodecurrent collectors for sodium sulphur cells.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention, a cathode currentcollector for a sodium sulphur cell is formed of conductive metal havinga surface onto which or onto at least part of which a layer of graphitefoil or graphite flakes has been isostatically pressed to force the foilor flakes to adhere to the surface.

The isostatic pressing of the graphite foil or flakes may be effectedusing a pressure in the range of 5,000 to 60,000 p.s.i. Typically apressure of 40,000 p.s.i. might be employed. Preferably the graphitefoil or flakes are pressed onto a rough surface. It has been found that,by such isostatic pressing onto a rough surface, it is possible toobtain a firmly adherent graphite layer which will withstand theconditions in the cathodic region of a sodium sulphur cell. Theisostatic pressing not only gives firm adherence to the substrate butprovides a further important advantage in that it densifies the graphitematerial. This material is homogenised and, for example, if graphitefoil is wrapped around a cylindrical surface, with an overlap of thefoil, then the isostatic pressing causes the overlapping portions to behomogenised.

It has been found that the surface of the conductive metal should have aroughness above one micron and preferably above five microns (thesebeing centre line averages on the recognised engineering scale).

The invention furthermore includes within its scope a method of making acathode current collector for a sodium sulphur cell comprising forming acollector of conductive metal, covering at least part of the surface ofthe collector with graphite foil or graphite flakes and subjecting theassembly to isostatic pressing to force the graphite foil to adhere tosaid surface. As stated above preferably the surface, to which thegraphite foil or flakes is applied, is a rough surface.

Preferably the conductive metal is aluminium but other metals may beemployed, for example, a nickel based alloy containing chromium andiron. Mild steel may be employed with a protective coating of aconductive material chemically inert to the cathodic reactant of asodium sulphur cell.

The metal surface may be roughened where the graphite foil is to beapplied. Such roughening may be effected by applying a surface coatingof a conductive material to the metal. This intermediate surface coatingis preferably a material resistant to corrosion in the cell environment.This coating may be a conductive carbide such as for example chromiumcarbide. Very conveniently however an aluminium or aluminium alloysubstrate may be coated with a nickel-chromium alloy by flame sprayingover the regions where the graphite foil or flakes are to be applied.

The invention includes within its scope a cathode current collector fora sodium sulphur cell made by the above-described method.

The invention furthermore includes within its scope a sodium sulphurcell having a cathode current collector as described above.

In one form of the invention, a sodium sulphur cell has a tubularelectrolyte element separating sodium around the outer surface of theelectrolyte element from cathodic reactant inside the tubularelectrolyte element and there is provided a cathode current collectorcomprising a conductive metal rod or tube arranged axially within theelectrolyte element, this metal rod or tube having a surface coveredwith graphite foil or graphite flakes isostatically pressed onto thesurface of the rod or tube.

It will be appreciated that the graphite foil or flakes need only beapplied over those parts of the surface of the current collector whichwould be exposed to the cathodic reactant when the cell is in use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates diagrammatically a sodium sulphur cell of the centralsulphur type;

FIG. 2 illustrates diagrammatically a sodium sulphur cell of the centralsodium type; and

FIG. 3 is a transverse section, to a larger scale, of a cathodecollector rod used in the cell of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In a sodium sulphur cell of the central sulphur type, as shown in FIG.1, a tubular member 10 of solid electrolyte material, typicallybeta-alumina, separates liquid sodium 11 around the outside of theelectrolyte tube from a cathodic reactant 12 comprising liquidsulphur/sodium polysulphides within the electrolyte tube. The sodium 11lies in an annular region between the tube 10 and an outer housing 13while the cathodic reactant 22 lies in an annular region between theinner surface of the tube 10 and an axially disposed current collectorrod 14.

A rod-shaped cathode current collector for use as the current collectorrod 14 of a sodium sulphur cell was made by wrapping a thin sheet 20(FIG. 3) of graphite foil (about 0.2 to 0.25 mm thick) around a rod 21of aluminium alloy having a surface coating 22 of flame-sprayednickel-chromium alloy. A sheath of soft polymeric material such asrubber or plasticized polyvinylchloride was then placed over thegraphite foil and the assembly was isostatically pressed by sealing thesheathed assembly in an outer pressure container, conveniently usingrubber O-rings as seals in a cylindrical assembly, and applying apressure of 40,000 p.s.i. hydraulically to the outer surface of thepolymeric sheath. This pressure caused the graphite foil to adhere tothe rough surface of the coated aluminium rod and it has been found thatgood adhesion can be obtained by this technique.

A rod made as described above was used as a cathode current collector ina sodium sulphur cell of the kind, such as has been described above,having a tubular solid electrolyte element with sodium around the outersurface of the electrolyte element and the cathodic reactant inside theelectrolyte element and the annular region between the electrolytesurface and the current collector rod. The cell, with the cathodecurrent collector formed as described above, was subjected to 100 chargeand discharge cycles over a period of 100 days at the operatingtemperature of about 350° C. At the end of this period no visible damageto the surface of the current collector rod could be seen after the cellhad been dismantled. No rise in electrical resistance of the cell wasobserved over this period. In another test, a group of five cells withcathode current collectors of aluminium having a flame-sprayed coatingof nickel-chromium alloy and with a 0.25 mm thick layer of isostaticallypressed graphite foil were on test for 345 days and completed 680 chargeand discharge cycles at an operating temperature of 350° C. with nochange in resistance.

Instead of using graphite foil, graphite flakes may be isostaticallypressed onto the rough surface of the substrate. The flakes are made toform a thin layer completely covering at least that part of the surfacewhich is exposed to the cathodic reactant in the cell.

If the cathodic reactant should, for any reason, penetrate the layer ofgraphite foil or flakes, the nickel-chromium layer forms a protectionfor the aluminium against chemical attack by the cathodic reactant.

Aluminium or aluminium alloy is the preferred substrate material becauseof its high electrical conductivity. Instead of using aluminium, itwould be possible to use other metals, e.g. a nickel-based alloycontaining chromium and iron. Mild steel may be used provided it has aprotective conductive coating, e.g. by chromising.

The invention is applicable to sodium sulphur cells of the centralsodium type as well as to cells of the central sulphur type. In a sodiumsulphur cell of the central sodium type, as shown in FIG. 2, thecathodic reactant 30 lies between the outer surface of an electrolytetube 31 and the inner surface of the housing 32. The sodium lies insidethe electrolyte tube. A similar isostatic pressing technique may be usedto press graphite foil 33 or graphite flakes onto the internal surfaceof the housing 32 which surface is roughened, e.g. by applying a surfacecoating of a chromium carbide material or nickel-chromium alloy.

We claim:
 1. In a method of making a cathode current collector for asodium sulphur cell comprising forming a collector of a conductive metaland coating the metal with a corrosion resistant material, wherein theimprovement consists of covering at least that part of the surface ofthe collector exposed to a corrosive material, with a materialconsisting of graphite foil and subjecting the assembly to isostaticpressing to force the graphite foil to adhere to said surface.
 2. Amethod as claimed in claim 1 wherein the conductive metal is aluminiumor an aluminium alloy.
 3. A method as claimed in claim 1 wherein theconductive metal is a nickel-based alloy containing chromium and iron.4. A method as claimed in claim 1 wherein the conductive metal is mildsteel with a protective coating of an electrically conductive materialchemically inert to the cathodic reactant of the sodium sulphur cell onwhich coating is applied the graphite foil.
 5. A method as claimed inclaim 2 wherein a surface coating of a conductive material is applied tothe aluminum or aluminum alloy to form a rough surface on the aluminumor aluminum alloy, the graphite foil being applied over said coating. 6.A method as claimed in claim 5 wherein the surface coating is of anickel-chromium alloy.
 7. A method as claimed in claim 1 wherein saidconductive metal has a rough surface onto which the graphite foil areapplied.
 8. A method as claimed in claim 7 wherein the rough surface isconstituted by an intermediate layer of conductive material onto whichthe graphite foil are applied.
 9. A method as claimed in claim 8 whereinsaid intermediate layer is a layer of material resistant to corrosion inthe cell environment.
 10. A method as claimed in claim 8 wherein theintermediate layer is a nickel-chromium alloy.
 11. A method as claimedin claim 8 wherein said intermediate material is a conductive carbide.12. A method as claimed in claim 1 wherein the isostatic pressing iseffected at a pressure in the range of 5,000 to 60,000 p.s.i.
 13. In amethod of making a cathode current collector for a sodium sulphur cellcomprising forming a collector of a conductive metal and coating themetal with a corrosion resistant material, wherein the improvementconsists of covering at least that part of the surface of the collectorexposed to a corrosive material, with a material consisting of graphiteflakes and subjecting the assembly to isostatic pressing to force thegraphite flakes to adhere to said surface.
 14. A method as claimed inclaim 13 wherein the conductive metal is aluminium or an aluminiumalloy.
 15. A method as claimed in claim 13 wherein the conductive metalis a nickel-based alloy containing chromium and iron.
 16. A method asclaimed in claim 13 wherein the conductive metal is mild steel with aprotective coating of an electrically conductive material chemicallyinert to the cathodic reactant of the sodium sulphur cell on whichcoating are applied the graphite flakes.
 17. A method as claimed inclaim 14 wherein a surface coating of a conductive material is appliedto the aluminum or aluminum alloy to form a rough surface on thealuminum or aluminum alloy, the graphite flakes being applied over saidcoating.
 18. A method as claimed in claim 17 wherein the surface coatingis of a nickel-chromium alloy.
 19. A method as claimed in claim 13wherein said conductive metal has a rough surface onto which thegraphite flakes are applied.
 20. A method as claimed in claim 19 whereinthe rough surface is constituted by an intermediate layer of conductivematerial onto which the graphite flakes are applied.
 21. A method asclaimed in claim 20 wherein said intermediate layer is a layer ofmaterial resistant to corrosion in the cell environment.
 22. A method asclaimed in claim 20 wherein the intermediate layer is a nickel-chromiumalloy.
 23. A method as claimed in claim 20 wherein said intermediatematerial is a conductive carbide.
 24. A method as claimed in claim 13wherein the isostatic pressing is effected at a pressure in the range of5,000 to 60,000 psi.